Steam-cracking of hydrocarbons

ABSTRACT

IN ORDER TO ELIMINATE THE DEPOSIT OF HARD MATERIALS, SUCH AS COKE, DURING THE FRACTIONATION OF PRODUCTS DERIVED FROM THE STEAM-CRACKING OF HYDROCARBONSTO PRODUCE OLEFINS, THERE IS PROVIDED A PROCESS WHEREIN A MIXTURE OF STEAM WITH A HYDROCARBON FEEDSTOCK OF THE NAPHTHA TO GAS OIL RANGE INCLUSIVE IS HEATED UP TO ABOUT 500-1200* C., THE RESULTING GASEOUS CRACKED PRODUCT IS ADMIXED SUCCESSIVELY WITH A FIRST RELATIVELY COOL STREAM OF RECYCLED IQUID HYDROCARBONS AND A SECOND RELATIVELY COOL STREAM OF RECYCLED HYDROCARBONS SO AS TO SUBJECT THE CRACKED PRODUCT TO A FIRST DECREASE OF TEMPERATURE DOWN TO ABOUT 195-230*C. AND CONDENSE A PART THEREOF, THE RESULTING LIQUID FRACTION IS SEPARATED FROM THE GASEOUS, UNCONDENSED FRACTION IN A FIRST FRACTIONATION ZONE, A PORTION OF THE SAID LIQUID FRACTION, AFTER PASSAGE THROUGH A FIRST INDIRECT HEAT EXCHANGE ZONE TO LOWER ITS TEMPERATURE, IS USED AS FIRST RECYCLED LIQUID HYDROCARBON STREAM, THE GASEOUS FRACTION WITHDRAWN FROM THE FIRST FRACTIONATION ZONE IS ADMIXED WITH A THIRD, RELATIVELY COOL STREAM OF RECYLED LIQUID HYDROCARBON, IN ORDER TO FURTHER LOWER THE TEMPERATURE OF THIS GASEOUS FRACTION DOWN TO ABOUT 140-190* C. AND CONDENSE ANOTHER PORTION THEREOF, THE RESULTING GASEOUS FRACTION IS SEPARATED FROM THE RESULTING LIQUID FRACTION IN A SECOND FRACTIONATION ZONE, A PORTION OF THE LATTER LIQUID FRACTION AFTER PASSAGE THROUGH A SECOND INDIRECT HEAT EXCHANGE ZONE TO LOWER ITS TEMPERATURE, IS USED AS THE SECOND AND THIRD RECYCLED HYDROCARBON STREAMS, AND THE GASEOUS FRACTION OF THE SECOND FRACTIONATION ZONE IS WITHDRAWN, SAID FRACTION CONTAINING THE DESIRED OLEFINS.

Aug. 3, 1971 5|GAHE ETAL 3,597,494

STEAM-CRACKING OF HYDRUGMIBONS Filed Sept. 4. 1969 3,597,494 STEAM-CRACKING UlF HYDRUCARBUNS Francois Bigache, Chatou, and Michel Lemee, Asnieres,

France, assignors to Institut lFraneais du lPetrole des Carburants et Luhrifiants Filed Sept. 4, 1969, Ser. No. 855,134 Claims priority, appliicztgorgilgrance, Sept. 45, 1968,

int. Cl. cm 3730; cro 9/36 US. Cl. 26tl683R 6 Claims ABSTRACT OF THE DiSCLQfilURE carbons and a second relatively cool stream of recycled hydrocarbons so as to subject the cracked product to a first decrease of temperature down to about 195-230 C. and condense a part thereof, the resulting liquid fraction is separated from the gaseous, uncondensed fraction in a first fractionation zone, a portion of the said liquid fraction, after passage through a first indirect heat exchange zone to lower its temperature, is used as first recycled liquid hydrocarbon stream, the gaseous fraction withdrawn from the first fractionation zone is admixed with a third, relatively cool stream of recycled liquid hydrocarbon, in order to further lower the temperature of this gaseous fraction down to about 140190 C. and condense another portion thereof, the resulting gaseous frac tion is separated from the resulting liquid fraction in a second fractionation zone, a portion of the latter liquid fraction after passage through a second indirect heat exchange zone to lower its temperature, is used as the second and third recycled hydrocarbon streams, and the gaseous fraction of the second fractionation zone is withdrawn, said fraction containing the desired olefins.

The steam-cracking processes are broadly used for converting hydrocarbon feedstocks to lighter products. In these processes, the feedstock is contacted with steam at a temperature between 500 and 1200 C., preferably 750 to 900 C.

The feedstock may be a pure or substantially pure compound, for example ethane, or a hydrocarbon cut, for example a naphtha, a gas oil, or a mixture thereof.

Various products may be obtained therefrom according to the type of the raw material and the severity of the treatment for example olefins such as ethylene, light gases, gasolines, fuels or heavier products.

The outflow from the cracking zone may be subjected to a quick chilling elfected by various means, for example by admixing the eflluent with a cool liquid or by indirect thermal exchange. However at this stage, the cracked material is maintained in the gaseous state in order to avoid the formation of coke deposits.

The cool effluent is usually subjected to a fractionation step in a separation column: gaseous products are removed at the top thereof. By cooling, 3 phases may appear: a gaseous phase containing the desired products, for example ethylene, an aqueous phase and an intermediary hydrocarbon phase, for example of the gasoline type. A portion of the hydrocarbon phase is usually refluxed to the separation column. From the base of this column there are collected condensed heavy products, usually contain ing monoand poly-cyclic aromatic compounds. Under 3,5WAM Patented Aug. 3, l97i the usual distillation conditions, these heavy products have a boiling point of approximately 200 C.

Deposits of hard materials, such as coke, may be formed at the bottom of this first separation column, which results in the plugging of the pipes, the stopping of the operation of the column and consequently that of the cracking furnace,

A main object of this invention is to avoid these inconveniences.

In conventional steam-cracking processes, particularly those applied to naphtha a portion of the heavy products from the bottom of the column is usually recycled, this portion being cooled down and thereafter admixed with the gaseous outflow from the cooling zone, and the resulting mixture is conveyed to the separation column. These heavy products withdrawn in the liquid state from the bottom of the column are at a relatively low temperature, for example about to C.

The cooling of the recycled portion of these heavy products usually permits to obtain steam at a relatively low pressure by indirect heat exchange, this steam being hardly usable in the process.

When the cracking process is applied to a material such as gas oil which is heavier than naphtha additional physical and chemical conditions are to be observed which change the operating conditions.

The dilution rate, i.e. the ratio of steam to feedstock at the inlet of the cracking furnace, must be higher in order to obtain the same yield of olefins.. This rate may be, for example, 0.5/1 by weight for a naphtha and from 1/1 to 2/1 for a light or heavy gas-oil, these figures being not limitative.

The temperature of the gaseous outflow from the cooling zone must be higher: 500 to 700 C. for a gas oil instead of 300 to 500 C. for a naphtha in order to avoid coke deposits at the beginning of the condensation stage.

The further cooling necessary to obtain the inlet temperature of the primary separation column is thus greater. It is thus more important to recover thermal energy at a satisfactory level in that case than in the case of the cracking of lighter materials.

Another object of this invention is to improve the recovery of thermal energy.

It is clear that the above inconveniences, particularly the formation of coke deposits in the column and the pipes, are maintained and very often enhanced.

Still another object of this invention is to crack heavy feedstocks while avoiding the above inconveniences.

According to this invention, the condensation of the heavy materials of the cooled cracking effluent is carried out in at least two separate zones having different average temperatures.

Thus this invention relates to a process for producing olefins, wherein a mixture of steam with a hydrocarbon feedstock is heated up to a high temperature, the resulting gaseous cracked product is admixed successively with a first relatively cool stream of recycled liquid hydrocarbons and a second relatively cool stream of recycled liquid hydrocarbons so as to subject the temperature of the cracked product to a first decrease and condense a part thereof, the resulting liquid fraction is separated from the gaseous, uncondensed fraction in a first fractionation zone, a portion of the said liquid fraction, after passage through a first indirect heat exchange zone to lower its temperature, is used as first recycled liquid hydrocarbon stream, the gaseous fraction withdrawn from the first fractionation zone is admixed with a third, relatively cool stream of recycled liquid hydrocarbon, in order to further lower the temperature of this gaseous fraction and condense another portion thereof, the resulting gaseous fraction is separated from the resulting liquid fraction in a second fractionation zone, a portion of the latter liquid fraction, after passage 3 through a second indirect heat exchange zone to lower its temperature, is used as the second and third recycled hydrocarbon streams, and the gaseous fraction of the second fractionation zone is withdrawn, said fraction containing the desired olefins,

The accompanying drawings illustrate this invention:

FIG. 1 is given for comparison purposes only and illustrates a conventional cracking unit.

FIG. 2 is an example of a unit operated according to the process of this invention, said process being welladapted for cracking heavy feedstocks.

A mixture of feedstock (for example naphtha or gas oil) introduced through line 1 and steam introduced through line 2 is passed through a cracking furnace 3 of the thermal or catalytic type. The ratio of water to hydrocarbons of the feedstock is higher than 0.5 by weight and usually between 0.65 and 10. The temperature is generally between 750 and 900 C. and the pressure between 1 and kg./cm. The outflow from the cracking furnace may be conveyed through line 4 to the cooling zone 5. The latter may consist of an indirect heat exchange device cooled by water, so as to produce steam. However the cooling zone 5 may be omitted and the cooling may be achieved by direct contact with the hydrocarbons of line 19.

The gaseous products, at a temperature of, for example, 300 to 500 C. (case of naphtha) or 500 to 900 C. (case of gas oil) are admixed with the hydrocarbons recycled through line 19. The resulting mixture, at a temperature of about 140 to 190 C., is introduced through line 6 in the separation column 7. From the top thereof there is recovered gases through line 8, the latter being cooled in 9 and conveyed through line 10 to the separating vessel 11 in which two liquid phases may be separated: the aqueous phase is withdrawn through line 12 and an intermediary hydrocarbon phase is partly recycled to the column 7 as reflux, through line 13, and partly withdrawn through line 14. The uncondensed gases evolve from the separating vessel 11 through line 15. The partial condensation of the gaseous outflow 8 may be achieved in several different manners.

From the bottom of the column there is recovered through line "16 heavy liquid products a part of which is withdrawn through line 17 and the other part recycled to the separation column through lines 18, 19 and 6 after cooling in the heat exchanger 20.

According to the temperature of the heavy products of lines 16 and 18, the exchanger 20 will be able or not to produce steam under a pressure useful on the industrial scale. Practically, these products are at a temperature of about 140 to 190 C. in line 16 and 100 to 150 C. in line 19.

It is thus clear that the condensation of the heavy products of the outflow from the cooling zone (line 6) is achieved only at the inlet of the separation column 7.

The same devices as those shown in FIG. 1 may be found in FIG. 2, i.e. the furnace, the cooling zone, the separation zone and others. All these devices are referenced by the same numerals as in FIG. 1. The outlet temperature of the heat exchanger 5, the inlet temperature of column 7 and the temperatures of lines 16, 17, 18 and 19 are those given hereabove. The temperature of pipe 26 is also about-100 to 150 C.

An additional device 21 is provided in the embodiment of FIG. 2, in order to condense and further partially separate heavy products at a temperature of about 195 to 230 C. The latter are withdrawn in part through line 22 at the said temperature, while another part of this liquid outflow is admixed with the gaseous outflow from the cooling zone after passage through line 23, the heat exchanger 24 and line 25. The outlet temperature of the heat exchanger 24 is about 165 to 200 C. In pipe 25, the temperature is about 150 to 190 C. In line 6, the temperature is about 195 to 230 C.

The outflow from the cooling zone, admixed with the recycled flows of lines 19, 25 and 26 and which has not been condensed in 21, is still conveyed to the separation column 7.

This results in the two-step condensation of the heavy products, the first one in device 21, the second one in column '7.

As pointed out before, a unit such as shown in FIG. 2 is adapted for cracking heavy feedstocks of the light or heavy gas oil type, i.e. feedstocks normally boiling over C. with a 10% distillation point over 200 C. and a final point over 250 C. It has also been pointed out that, in such a case, the outflow from the cooling zone (i.e. the gaseous product issued from device 5) is usually at a high temperature, for example about 500 to 900 C., preferably 550 to 650 C.

After the cooling of this efliuent by admixture with the products recycled through lines 25 and 26, there is condensed in 21 a part of the heavy products. The latter are discharged from 21 through line 22 at a relatively high temperature, for example about to 230 C. A portion is recycled through line 23 and the heat exchanger 24. Since the heavy products obtained at this stage (line 22) are at a high temperature, there is obtained, in exchanger 24, steam at a moderate pressure (about 10 kg./crn.

This vapor is valuable since it may be used directly for cracking the feedstock.

Lighter feedstocks such as naphthas may be also subjected to the treatment of this invention.

From the separation column, there is recovered, at the top, gases which may be partly condensed, and, at the bottom, the other heavy products which are discharged from the column through line 16. The liquid flowing in line 16 is at a relatively low temperature. A part of the heat contained therein may be recovered by means of the exchanger 20; but it is not possible at this stage to obtain steam under relatively high pressure. This is not an inconvenience since the latter has already been produced in the exchanger 24.

In a conventional unit such as that shown in FIG. 1, used for treating a heavy feedstock, it is not possible either to produce steam under high pressure in the exchanger 20 for the above stated reasons.

In addition to the recovery of heat at a high level of energy, this invention offers other advantages. It has been shown that the outflow from the cracking zone contains solid particles such as coke which, in conventional processes, are deposited on the lower plates of column 7.

In the process of this invention, these solid particles are more selectively deposited in device 21. This is not inconvenient since this condensation device 21 has usually no plate (this is commonly a flask) so that it can be easily cleaned up. It is also possible to make use of several devices such as 21 in parallel so that the operation of one or more thereof is stopped for cleaning while at least one other is on run without requiring the stopping of the operation of the separation column 7.

However, if solid deposits are formed in column 7, these deposits will be much smaller than in the case where is omitted appartus 21 (condensation zone for the heavy products).

It must be pointed out that the temperatures given herebefore are illustrative and by no way limitative. Since the steam-cracking may be carried out within a broad temperature range even for a feedstock of a given type, it is obvious that the temperatures at the different steps are partly dependent on this cracking temperature.

Other embodiments of the process may be contemplated without departing from the scope of this invention. For example, the distillation column may be operated with a water reflux at its upper part.

On line 6, separate connection points of lines 26 and 25 for the recycled liquids may be provided for, instead of the common one of FIG. 2.

The liquids recycled through lines 26 and 23 do not have exactly the same use.

It can be said, for sake of brevity, that the heavy liquid from the separating vessel 21 runs in a circle through line 22, line 23, the cooling vessel 24, lines 25 and 6 and the device 21 while remaining in the liquid state. It withdraws from the chilled outflow a large amount of heat at a high level, the latter being recovered in the exchanger 24. Simultaneously it contributes to condense in 21 a part of the cooled heavy products.

The recycling of the heavy liquid of line 26 also contributes to cool down the gaseous outflow from the cooling zone. It also vaporizes at least in part, which increases the dew point of the mixture introduced in device 21, thus the temperature of the liquid of line 22 and also the amount of heat at a high temperature which is available in the exchanger 24.

These two recyclings are necessary for obtaining all of the advantages of this invention, i.e. particularly in the case of heavy feedstocks: the recovery of heat at a high temperature level and the reduction and sometimes the stoppage of the formation of deposits in the separation column.

By way of illustration, not of limitation, the ratio of the liquid feed rates through lines 23 and 26 is preferably from 10/1 to 30/1 respectively, although ratios from 2/1 to 50/1 are convenient.

These ratios by weight are advantageously selected between /1 and 30/1 (pipes 23 and 4), between 0.2 and 2 (pipes 26 and 4) and between 0.1 and 2 (pipes 19 and 4).

The ratio by weight of the heavy products condensed in the two zones, i.e. in apparatuses 21 and 7, may be selected, for example, between 0.5 and 2 and preferably at about 1.

By way of non limitative example, there has been treated a gas oil having a specific gravity of 0.882, distilling between 166 and 435 C. with a distillation point of 240 C., using the scheme of FIG. 2. The ratio by weight of water to hydrocarbons was 1.1, the temperature 855 C. and the outlet pressure 2.2 kg./cm. The product withdrawn from exchanger 5 was at a temperature of about 650 C. The average temperature of column 21 was 215 C., that of column 7 was 186 C. Steam at 180 C. was produced in unit 24 and at 150 C. in unit 20.

The yield of ethylene was 21% by weight (without recycling at the furnace inlet).

The ratio of the feed rates in pipes 23 and 26 was 20.

As a rule, from 10 to 60% of the cracking hydrocarbon outflow is condensed in 21 and from to 60% in 7, the uncondensed hydrocarbon gases amounting to 5 to 60% of this outflow, these values being by weight.

What we claim is:

1. A process for producing olefins, wherein a mixture of steam with a hydrocarbon feedstock of the naphtha to gas oil range inclusive is heated up to about 5500-1200 C.,

the resulting gaseous cracked product is admixed successively with a first relatively cool stream of recycled liquid hydrocarbons and a second relatively cool stream of recycled hydrocarbons so as to subject the cracked product to a first decrease of temperature down to about 195-230 C. and condense a part thereof, the resulting liquid fraction is separated from the gaseous, uncondensed fraction in a first fractionation zone, a portion of the said liquid fraction, after passage through a first indirect heat exchange zone to lower its temperature, is used as first recycled liquid hydrocarbon stream, the gaseous fraction withdrawn from the first fractionation zone is admixed with a third, relatively cool stream of recycled liquid hydrocarbon, in order to further lower the temperature of thsi gaseous fraction down to about 190 C. and condense another portion thereof, the resulting gaseous fraction is separated from the resulting liquid fraction in a second fractionation zone, a portion of the latter liquid fraction after passage through a second indirect heat exchange zone to lower its temperature, is used as the second and third recycled hydrocarbon streams, and the gaseous fraction of the second fractionation zone is withdrawn, said fraction containing the desired olefins.

2. A process according to claim 1, wherein the ratio of the feed rate of the first recycled liquid stream to the feed rate of the second recycled liquid stream is 10:1 to 30:1 respectively, and the ratio of the feed rate of the first recycled liquid stream to the feed rate of the third recycled liquid stream is 10:1 to 50:1 respectively.

3. A process according to claim 1, wherein the ratio by weight of water to hydrocarbons is between 0.5 and 10 and the cracking temperature between 750 and 900 C.

4. A process according to claim 1, wherein the ratio by weight of the heavy products condensed in the first and the second fractionation zones is between 0.5 and 2.

5. A process according to claim 1, wherein the ratio by weight of the respective feed rat-es with respect to the cracking outlet stream is between 5/1 and 30/1 for the first recycled stream and between 02/1 and 0.2/1 for the second recycled stream and between 0.1/1 and 2 for the third recycled stream.

6. A process according to claim 1, wherein the pressure is between 1 and 5 atmospheres, and said mixture is heated up to 750-900 C.

References Cited UNITED STATES PATENTS 9/1958 Smith et al 196- 19 7/1960 Pardee 260--683 US. Cl. X.R. 

